In a conventional power steering system, an engine driven pump outputs a fixed volume of fluid per engine revolution. Thus, the flow rate of the pump is proportional to the engine speed. The pump is sized to provide the necessary flow so that an operator can easily steer the vehicle when the engine speed corresponds to highway cruising. An electric motor driven pump may be used to make up for a flow deficit at low engine speeds. However, the engine driven pump and the electric motor driven pump may not provide enough flow to permit turning of the steerable wheels when the vehicle is heavily loaded and motionless.